Let total manwork required to build the table = L.C.M. (40,60) = 120 units P, Q and R together can make the table in 40 minutes. ⇒ Total efficiency of P, Q and R together =
120
40
=3 units/min Similarly, (P+Q)'s efficiency =
120
60
=2 units/min ⇒ R's efficiency = 3−2=1 unit/min ∴ Time required by R alone to build the table =